2. Field of the Invention
The present invention relates to logging of a subterranean formation for determination of density using gamma rays. Particularly, this invention relates to determination of formation density without positioning the logging probe against the wall of the borehole traversing the earth formation. More particularly, this invention is useful for measurement of density while drilling.
2. Setting of the Invention
Wireline gamma ray density probes are devices incorporating a gamma ray source and a gamma ray detector, shielded from each other to prevent counting of radiation emitted directly from the source. During operation of the probe, gamma rays (or photons) emitted from the source enter the formation to be studied, and interact with the atomic electrons of the material of the formation by photoelectric absorption, by Compton scattering, or by pair production. In photoelectric absorption and pair production phenomena, the particular photons involved in the interacting are removed from the gamma ray beam.
In the Compton scattering process, the involved photon loses some of its energy while changing its original direction of travel, the loss being a function of the scattering angle. Some of the photons emitted from the source into the sample are accordingly scattered toward the detector. Many of these never reach the detector, since their direction is changed by a second Compton scattering, or they are absorbed by the photoelectric absorption process of the pair production process. The scattered photons that reach the detector and interact with it are counted by the electronic equipment associated with the detector.
The major difficulties encountered in conventional gamma ray density measurements include definition of the sample size, limited effective depth and sampling, disturbing effects of undesired, interfering materials located between the density probe and the sample and the requirement that the probe be positioned against the borehole wall. The chemical composition of the sample also affects the reading of conventional gamma ray density probes.
One prior art wireline density probe disclosed in U.S. Pat. No. 3,202,822 incorporates two gamma ray detectors, one collimated gamma ray source and ratio building electronic circuits, and is useful as long as the interfering materials, located between the detectors of the probe and the formation sample, are identical in thickness and chemical composition along the trajectories of emitted and received gamma rays. Non-uniformities in the wall of the borehole will interfere with the proper operation of the probe. Such non-uniformities can be caused by crooked holes, by cave-ins, and by varying thicknesses of the mudcake on the wall of the hole.
The prior art also includes U.S. Pat. No. 3,846,631 which discloses a wireline density probe which functions regardless of the thickness and the chemical composition of materials that are located between the density probe and the sample. The method comprises passing of two gamma ray beams from two intermittently operated gamma ray sources into the sample, receiving the radiation backscattered from each of the two sources by two separate detectors, and building ratios of products of the four separate counting rates in such a manner that the numerical result is an indication of the density of the sample.
The critical dimension of the two-detector probe is the spacing between the detectors. If the interfering materials are non-uniform over distances comparable to the spacing of the two detectors, the measured density will be erroneous.
Neither of the wireline probes described above is disclosed as being useful for measurement while drilling and incorporation into a rotating drill string.